Probing a Brown Dwarf’s Atmosphere

by Paul Gilster on January 9, 2013

The American Astronomical Society’s meeting in Long Beach is going to occupy us for several days, and not always with exoplanet news. Brown dwarfs, those other recent entrants into the gallery of research targets, continue to make waves as we learn more about their nature and distribution. The hope of finding a brown dwarf closer than Alpha Centauri has faded and recent work has emphasized that there may be fewer of these objects than thought — WISE data point to one brown dwarf for every six stars. But habitable planets around brown dwarfs are not inconceivable, and in any case we are continuing to build the census of nearby objects.

The latest from AAS offers up what could be considered a probe of brown dwarf ‘weather.’ If the idea of weather on a star seems odd, consider that the cooler brown dwarfs are far closer to gas giants than stars, unable to trigger hydrogen fusion and gradually cooling as they age. That means cloud patterns form and huge storms plow through the various atmospheric layers. At AAS, Daniel Apai (University of Arizona) presented the results of work on the brown dwarf 2MASSJ22282889-431026, which he conducted with a team led by the university’s Esther Buenzli. The results are useful not just for brown dwarf study but planetary atmospheres as well.

Using the Hubble and Spitzer space telescopes simultaneously, the researchers found that every ninety minutes the light from the star varied as it rotated. Because they were looking at the object at different wavelengths, they were able to see that the timing of the brightness change depended on wavelength. Some infrared wavelengths emerge from deep within the star, while others are blocked by water vapor and methane at higher altitudes. What we’re getting, in other words, is a look at layers of material being carried around the brown dwarf in likely storms.

But these aren’t your usual clouds, according to Mark Marley (NASA Ames), a co-author on the paper:

“Unlike the water clouds of Earth or the ammonia clouds of Jupiter, clouds on brown dwarfs are composed of hot grains of sand, liquid drops of iron, and other exotic compounds. So this large atmospheric disturbance found by Spitzer and Hubble gives a new meaning to the concept of extreme weather.”

The brown dwarf in question is cool in stellar terms but still hot enough — 600 to 700 degrees Celsius — to produce clouds like those Marley describes. The light variations offer the researchers a chance to understand the brown dwarf’s weather in the vertical dimension. Another scientist involved in the work is Adam Showman, also at the University of Arizona, who notes: “The data suggest regions on the brown dwarf where the weather is cloudy and rich in silicate vapor deep in the atmosphere coincide with balmier, drier conditions at higher altitudes — and vice versa.”

Image: This graph shows the brightness variations of the brown dwarf named 2MASSJ22282889-431026 measured simultaneously by both NASA’s Hubble and Spitzer space telescopes. As the object rotates every 1.4 hours, its emitted light periodically brightens and dims. Surprisingly, the timing, or phase, of the variations in brightness changes when measured at different wavelengths of infrared light. Spitzer and Hubble’s wavelengths probe different layers in the atmosphere of the brown dwarf. The phase shifts indicate complex clouds or weather patterns that change with altitude. Credit: NASA/JPL-Caltech.

A way forward in brown dwarf studies is suggested in the paper:

We have measured longitudinal variability throughout the near-infrared in a T6 dwarf and found an unusual correlation of light curve phase with the pressure probed by a given wavelength, which suggests a complex horizontal and vertical atmospheric structure. Our observations should provide an incentive to drive the development of higher-dimensional atmospheric models in order to gain a deeper understanding of dynamical and radiative processes in brown dwarf and exoplanet atmospheres.

Hubble and Spitzer are thus giving us the ability to probe a brown dwarf atmosphere in ways that, according to Apai, are not dissimilar to how doctors probe the body with medical imaging techniques. The unexpected offset between the different layers of atmospheric material tells us that a feature we might see on the surface of a brown dwarf may have shifted as we push into the inner layers of the object. That’s a sign of wind-driven clouds in constant motion, a warmer version of features like Jupiter’s Great Red Spot on an object not quite planet, not quite star.

This release is a nice companion to arxiv paper http://arxiv.org/pdf/1301.1669. In short, if ~3-6 jupiter mass Y Dwarf (planemo?) WISE 1828+2650 is truly at the tail end of normal star formation processes, then the space denisty of brown dwarfs will be low. On the other hand, if its at the massive end of a population of ejected free floating ‘normal’ planets, then there could be billions of them out there (just as a microlensing survey has suggeted) – maybe one closer than Alpha cen.
We need bigger light buckets though, above the atmosphere. I hope after all the financial/political pain, JWST does us all proud!

That numerical distribution (1 brown dwarf for every 6 stars) is surprising. I wonder if that is their estimate for our entire galaxy’s populations of brown dwarfs vs. stars, or for our local region (perhaps our spiral arm)? Also, the cold (Class Y, I believe they’re called) brown dwarfs emit virtually no radiation, so there could be enormous numbers of them out there (perhaps even nearby), undetected.

great article. We are just getting enough data of high quality to do these studies. The JWST will help in chracterizing brown and red dwarfs that have been identified, and will make sense of a lot of the physics of these interesting objects. I do not think that it will help ( much) in the discovery of new objects, given that is is a point and study telescope with incredible power but relatively small field of view (FOV). We still need a large survey scope , and while gaia will help at visible and near very near infrared, we still do not have survey capacity in the infrared spectrum. Working in the 3 to 12 micron range ( ” hot tea to winter’s day on mars” ) range is critical to understand planet formation and to the discovery of rogue planets and older brown dwarfs. We have to extrapolate to uncomfortable limits based on today’s WISE data. More Wise data would help ( restart the telescope , it is still functional!). But we also need to build a new truly infrared scope with a 1 meter-plus light bucket , a 4 to 10 square degree FOV and a five year mission! These cool objects need a bigger mirror because the signal is faint. We could then talk about large objects in the Oort cloud, about the real distribution of brown dwarfs and about matter distribution in the early universe- Oh and yes, planet and star formation all across the Galaxy. As a bonus we get more dark asteroids and trojan objects and who knows what other cosmic exotica!

Among the hundred billion stars which can be observed in the Milky Way, there is a group of stars, the so-named ultra-cool dwarfs, defined as stars with a temperature below 2500 K, which includes ultra-cool dwarfs and brown dwarfs. It is a really interesting group: they are the most ancient objects in our galaxy and, therefore, they can provide information about its primitive chemical composition. This is one of the objectives of the Gaia mission, which will be launched at the end of 2013 by the European Space Agency.

When observing them, they seem quite similar, but there are clear differences between brown dwarfs and ultra-cool dwarfs: brown dwarfs do not reach the temperature they need to produce the nuclear reactions which characterize ultra-cool dwarfs. It could be said that brown dwarfs are failed stars because they lack mass.

A study published in the journal Astronomy & Astrophysics, led by the National University of Distance Education (UNED, Spain) and in which researchers from the Institute of Cosmos Sciences of the University of Barcelona (ICCUB) participated, has developed a method that will allow Gaia to detect tens of ultra-cool dwarfs in the Milky Way. The method to estimate physical parameters of these objects, such as temperature or gravity, has also been validated. Researchers have used data-mining techniques to make estimations taking into account the parameters that Gaia can measure and its design characteristics.

This method highly improves the accuracy of data, so the estimation of ultra-cool dwarfs’ temperature will have little margin of error. This improvement in accuracy will also increase considerably the number of ultra-cool dwarfs detected. However, only few tens of those which show a temperature lower than 1500 K and weak brightness in the wavelengths observed by Gaia could be detected.

The National University of Distance Education, the Astrobiology Center (CSIC-INTA), the University of Cï¿œdiz, and the German-Spanish Calar Alto Observatory participated in the research too.

Gaia Mission

Gaia will conduct a census of a thousand million stars in our galaxy, monitoring each of its target stars about 70 times over a five-year period. It will precisely chart their positions, distances, movements, and changes in brightness. The launch of the satellite, currently on trial, will take place at the end of 2013, on a Soyuz-Fregat rocket. During this period of time, it is expected to discover hundreds of thousands of new celestial objects. Gaia instruments are so precise that, if it was on the Earth, it would be able to measure the thumb of a person who is on the Moon’s surface.

Gaia will provide information about different parameters (position, speed, distance, physical properties, etc.) for each object, so a map of our galaxy will be obtained. Gaia mission will generate more than one petabyte of information, in other words, one million gigabytes, which will have to be processed and analyzed in order to get results.

About 400 scientists collaborate in the project in which thirty researchers and technicians from the Department of Astronomy and Meteorology of the UB and from the ICCUB had an important role in creating and designing the instrument, as well as in processing and simulating the data to be obtained during the mission.

Charter

In Centauri Dreams, Paul Gilster looks at peer-reviewed research on deep space exploration, with an eye toward interstellar possibilities. For the last nine years, this site has coordinated its efforts with the Tau Zero Foundation, and now serves as the Foundation's news forum. In the logo above, the leftmost star is Alpha Centauri, a triple system closer than any other star, and a primary target for early interstellar probes. To its right is Beta Centauri (not a part of the Alpha Centauri system), with Beta, Gamma, Delta and Epsilon Crucis, stars in the Southern Cross, visible at the far right (image: Marco Lorenzi).

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